A catch-all word that can express anything from extreme joy, to indignation, to sarcasm (all depending on the tone of voice and inflection used). Its most common use is for expressions of happiness or approval and is generally said twice in a row, as in "neenuu neenuu."

Jane: "Hey, Joe! I just baked a ton of super-delicious, double chocolate chip cookies! Would you like a big plate of them?"
Joe: "What kind of question is that?! I love your cookies! NEENUU NEENUUU!!!"